Pass definition as CXX_FLAGS for GTest ExternalProject
Pass definition as CXX_FLAGS for GTest ExternalProject

This addresses the following CMake warning.
CMake Warning at C:/Program Files
(x86)/CMake/share/cmake-3.6/Modules/ExternalProject.cmake:1131
(message):
  Line 'CMAKE_CXX_FLAGS=-DGTEST_HAS_TR1_TUPLE=0' does not match regex.
  Ignoring.

SimpleITK/itkSimpleITKFiltersModule,SimpleITK/itkSimpleITKFiltersModule